Magnetorheological fluids typically comprise magnetically responsive particles suspended in a base fluid. A third element, known as an additive, may also be included to assist in suspending the particles and preventing agglomeration. In the absence of a magnetic field, the magnetorheological fluid behaves similar to a Newtonian fluid. However, in the presence of a magnetic field the particles suspended in the base fluid align and form chains which are roughly parallel to the magnetic lines of flux associated with the field. Further, the magnetic field causes the fluid to enter a semi-solid state which exhibits increased resistance to shear. Resistance to shear is increased due to the magnetic attraction between particles of the chains. Adjacent chains of particles combine to form a sealing wall. The effect induced by the magnetic field is both reversible and repeatable. Electrorheological fluids are analogous, although responsive to an electric field rather than a magnetic field. However, field-responsive fluids have some drawbacks.
The use of field-responsive fluids in long fluid columns such as those found in wellbores can cause problems because the specific gravity of fluid is typically higher than commonly used fluids and for magnetorheological fluids on the order of 3-4. As a result, the hydrostatic pressure exerted at lower sections of the long fluid column can reach values great enough to damage equipment and completion. One reason for the relatively great specific gravity of magnetorheological fluids is that the magnetic properties which enable the field-responsive particles to function are found in materials having relatively higher densities than many fluids, e.g., iron and nickel. Some examples of magnetorheological particle technology known in the art include a method of manufacturing shaped magnetic particles published in Deshmukh, S.S., “Development, characterization and applications of magnetorheological fluid based ‘smart’ materials on the macro-to-micro scale,” MIT PhD Thesis, 2007; and polymer coated magnetic beads sold under the trade name DYNABEADS® by Invitrogen Corporation for cell separation and expansion applications.
Another drawback of field-responsive fluids is susceptibility to creep flow. Creep flow refers to the tendency of fluid to traverse the chains of particles by passing through spaces between particles. For example, a magnetorheological fluid shaft seal utilizes a magnetic field supplied between two segments of a housing structure to cause the fluid to form a semi-solid seal in the gaps between the housing and shaft. This seal functions whether or not the shaft is rotating, and also exhibits shear resistance which can counter differential pressure, i.e., pressure inside the housing versus pressure outside the housing. However, differential pressure may still cause fluid creep through the spaces between magnetically responsive particles. In other words, even if the magnetic forces are sufficient to resist the shearing force due to differential pressure load, the base fluid is free to flow through the crevices between magnetorheological particles. This can lead to an undesirable case where fluid loss or gain occurs in the chamber that is to be sealed. Park, J. H, Chin, B. D., and Park, O. O., “Rheological Properties and Stabilization of Magnetorheological Fluids in a Water-in-Oil Emulsion,” Journal of Colloid and Interface Science 240, 349-354, 2001,describes shear properties of a magnetorheological fluid with a water-in-oil emulsion base.